The present invention relates to a honeycomb catalytic article for trapping particulate matter in exhaust gas.
In consideration of the influences on the environment, there is increased need to remove particulate matter contained in exhaust gas discharged from internal combustion engines such as an automobile engine, a construction machine engine, and a stationary engine for industrial machines; burning appliances, and the like. In particular, regulations regarding the removal of particulate matter (PM) discharged from a diesel engine tend to be strengthened on a global basis. From such circumstances, a diesel particulate filter (DPF) for trapping and removing PM is in the spotlight.
As a mode for a DPF, there is a honeycomb filter provided with a porous partition wall matrix separating and forming a plurality of cells functioning as fluid passages, where predetermined cells each of which has an open end portion on one side and a plugged end portion on the other side and remaining cells each of which has a plugged end portion on the one side and an open end portion on the other side are alternately disposed, and a fluid (exhaust gas) flowing in from the end portion on the one side where the predetermined cells are open passes through the partition walls, flows out on the remaining cell side as a permeated fluid, and further flows out from the other end portion on the other side where the remaining cells are open, thereby trapping and removing PM in exhaust gas. Since a filter (wall-flow type filter) having a structure where exhaust gas permeates porous partition walls like the above honeycomb filter can have a large filtration area, filtration flow rate (partition wall permeation flow rate) can be made low, and pressure loss is small with relatively good trapping efficiency of particulate matter. The particulate matter depositing on the DPF contains carbon as the main component, and there is employed a method for oxidizing and removing fine particle substances depositing with a certain period by sending high temperature gas containing oxygen or oxygen and nitrogen dioxide into the DPF. In order to perform it effectively, it is known that loading of a catalyst inside partition wall pores and on the surface of the partition walls of the DPF is helpful (see, e.g., JP-A-2003-293730).
However, a DPF where such a honeycomb filter is applied has the following problems to be solved.
(a) When trapping of PM is started in a clean state, PM enters pores of the porous partition walls in the first place to be in a state of deep bed filtration where PM is trapped inside the partition walls and a state of surface filtration where PM is trapped on the surface of the partition walls. After the PM deposits on a surface of the partition walls, the filtration is moved to a cake filtration where the PM forms a layer functioning as a filter. In such a filtration process, PM deposits inside (in pores of) the partition walls in the initial deep bed filtration stage. Therefore, right after the start of trapping PM, substantial porosity of the partition walls may fall to speed up the flow rate of exhaust gas passing through the partition walls, and thereby pressure loss may rapidly rise. Such rapid rise in pressure loss right after the start of trapping PM is not preferable because it causes deterioration in engine performance.
(b) In a DPF, it is necessary to combust and remove PM for regeneration when a certain amount of PM is trapped and deposited. In this case, generally, the amount of deposited PM is estimated from the pressure loss. However, in a DPF having such a conventional partition wall pore structure, since only PM deposited in pores is naturally combusted, pressure loss has hysteresis with respect to the whole PM deposition amount, and it is impossible to estimate the PM amount with high accuracy on the basis of pressure loss.
(c) A DPF having smaller pore sizes of the partition walls and thicker partition walls can trap PM more effectively. In addition, a smaller pore size of the partition walls is preferable in order to inhibit entry of PM into the partition walls (pores) and move the filtration to the cake filtration early. However, it is not desirable to reduce the pore size of the partition walls and increase the thickness of the partition walls because it causes increase of pressure loss of the partition walls themselves before PM deposition to deteriorate engine performance.
(d) In a DPF, there is a case of employing two-layer structure where a surface layer is formed on the porous partition wall matrix. However, conventionally, there is a case that the surface layer is peeled to deteriorate filtration accuracy.
(e) In a DPF, there are cases that a catalyst is loaded in pores of the partition walls and on a surface of the partition walls to raise the PM oxidation rate, that PM oxidation initiation temperature is lowered to reduce the PM deposition amount, and that regeneration time is shorten, or regeneration temperature is lowered to save fuel. However, since there are many cases that the PM (fine particle substance) deposits more on the inlet side of the partition walls without reaching the vicinity of the surface on the outlet side. Therefore, a catalyst in the partition walls cannot be used effectively.
(f) When a catalyst layer is formed only on the partition wall surface layer in order to solve the problem of (e), partition wall passage resistance of gas increases too much to cause increase in pressure loss in the DPF. As a result, deteriorations in engine output and fuel cost performance occur.